1. Field of the Invention
The invention relates to a ball screw unit and an electric power steering apparatus.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 2006-256414 describes a rack-assist electric power steering apparatus (EPS) that includes a hollow shaft in which a rack shaft is inserted and which is rotated by a driving motor, and that converts rotation of the hollow shaft into reciprocation of the rack shaft using a ball screw unit, thereby supplying an assist force to a steering system.
In such an EPS, the ball screw unit is formed by arranging a plurality of balls that serve as rolling elements in a spiral rolling-element path formed by aligning a thread groove formed in an outer periphery of the rack shaft with a thread groove formed in an inner periphery of a ball screw nut.
When being arranged in the rolling-element path, the balls are interposed between the rack shaft and the ball screw nut. When the ball screw nut rotates relative to the rack shaft, each ball rolls within the rolling-element path while receiving a load. The ball screw unit has a return path that serves as a shortcut between two points set within the rolling-element path, that is, an upstream-side point and a downstream-side point. The balls that have rolled through the rolling-element path pass through the return path to be returned from the downstream-side point to the upstream-side point.
Each of the balls that roll within the rolling-element path is infinitely circulated through the return path, whereby the ban screw unit converts rotation of the ball screw nut into axial movement of the rack shaft. JP-A-2006-256414 describes the EPS that is configured to convert a motor torque into an axial assist force and transmit the assist force to the rack shaft when the ball screw nut fixed to an inner periphery of a motor shaft rotates together with the motor shaft.
Downsizing of components of an EPS has been strongly demanded. In order to downsize components of an EPS, a configuration in which a return path is formed by fitting a circulation member in fitting holes that pass through a ball screw nut in the radial direction is usually adopted in a ball screw unit for the EPS.
For example, as described in JP-A-2007-78093, a pair of fitting holes corresponding to two points set within a rolling-element path and a return hole that passes through a ball screw nut in its axial direction and that crosses each fitting hole are formed in the ball screw nut. A circulation member that is fitted in the fitting holes connects the return hole to the rolling-element path, whereby a return path is formed.
However, in the configuration in which the return path is formed by the circulation member that is fitted in the fitting holes, it may be difficult to properly fit the circulation member in the fitting holes. If an error in a fitting position of the circulation member occurs due to, for example, inclination of the circulation, member that may be caused when the circulation member is fitted, a step is formed in a connection portion between the circulation member and the fitting hole within the return path. This step may hinder smooth circulation of the balls within the return path, and abnormal noise or vibration may be generated.
In order to properly fit the circulation member in the fitting holes, in the ball screw unit described in JP-A-2007-78093, a positioning pin is formed on the circulation member, and a positioning hole corresponding to the positioning pin is formed on the fitting hole side. In the fitting hole, there is formed an engagement groove in which a C-shaped ring that forms an engagement element is fitted. Because the positioning pin is fitted in the positioning hole, it is possible to assure proper fitting of the circulation member. In addition, because the circulation member is restrained by the C-shaped ring that is fitted in the engagement groove, it is possible to suppress occurrence of displacement of the circulation member after fitting of the circulation member.
However, a ball screw unit used in an EPS is a compact member. Therefore, the above-described positioning pin and C-shaped ring are considerably small members, and the positioning hole and engagement groove corresponding to the positioning pin and C-shaped ring are also considerably small. Due to the low workability and assembling efficiency, applying this configuration to an EPS has a cost disadvantage.